The present invention relates to a construction of electronic tuning type tuners for television or the like and, more particularly, to a construction suitable for a tuning circuit including a resonance rod.
An electronic tuning type tuner, for instance UHF electronic tuning type tuner, has a construction having a plurality of cavities defined by shield plates, with electronic component parts such as transistors, resistors, capacitors and resonance rods being disposed in these cavities. In tuners recently proposed, there is a trend toward using two printed circuit boards overlapped back to back. FIGS. 1 and 2 show such a tuner construction. As is shown, a cavity is defined in a chassis 1 by shield plates 2, and in this cavity, electronic component parts such as resonance rod 3, a varactor diode 4, an end capacitor 5 and a resistor 6 are provided to form a tuning circuit. The resonance rod 3, resistor 6 and other electronic parts are soldered to a printed circuit board lamination 7 including two printed circuit boards laminated back to back. The printed circuit board lamination consists of upper and lower printed circuits boards 7a and 7b which are adhered together by adhesive 8. A printed pattern 7c is provided on board 7a for supplying a tuning voltage to the varactor diode 4, a pattern 7d is provided on board 7b for connecting the resistor 6 and resonance rod 3, and a pattern 7e is provided on board 7b for grounding.
With such construction of the resonance rod circuit, a lead 4a of the varactor diode 4 is considerably long since it must extend through the two printed circuit boards 7a and 7b. Therefore, the residual inductance is high, so that the frequency range is narrow. If the capacitance of the end capacitor 5 is increased to overcome this drawback, the circuit Q is reduced. In addition, the dielectric constant of the printed circuit board 7a is changed with changes of the temperature and relative humidity, so that the resonant frequency of the circuit is subject to fluctuations.